As one of the sensors for detecting a low molecular weight organic liquid like gasoline, there is such a sensor in which a conductive sheet prepared by dispersing a conductive carbon on a liquid-absorbing rubber sheet, etc. that swells upon absorption of liquid, is kept constantly conductive, and the sensor responds to an increased electric resistance attributable to the widened distance between carbon particles which is caused by swelling of the conductive sheet when it contacts gasoline, etc. The applicant of the present invention has proposed a highly sensitive liquid detection sheet-form sensor superior in swelling property as compared with conventional sensors upon contact with a liquid or its vapor to be sensed (U.S. Pat. No. 5,173,684).
The sheet-form sensor such as the above is useful when used as a liquid-leakage sensor installed in a place where liquid-leakage is predictable, that is, when used as a point type sensor. However, in cases where installation of many sensors to cover a wide area is required because of unpredictable points of liquid-leakage, that is, when area type sensing is necessary, there arises a problem of being unable to build an industrially practical detection system because of the likelihood of high cost for installing the above sheet-form sensors throughout the entirety of the target area.
It is desirable that a conductive layer for sensing a liquid should be formed into a thin layer to have high sensitivity. When the above sheet form sensor is formed into a thin layer, however, it is necessary to adhere a reinforcing sheet on one face of the layer to secure mechanical strength, and there also arises a problem of lowered detection sensitivity on the side of the reinforcing sheet.
Moreover, Japanese Utility Model Publication Jitsukou Sho 54-15672 (1979) describes use of a resistor wherein conductive powder is dispersed on epoxy resin, etc. as a sensing element for detecting oil leakage. Yet, the resistor made of such materials is disadvantageous in that it has poor processability such as extruding property, and a secondary processing for forming a desired shape is difficult.
At a gasoline station or the like, liquid leakage is detected by inserting a gaging bar into liquid-leakage detection holes having a diameter of about 30 mm which are provided on a periphery of an underground tank about once a week, and checking liquid-leakage by observing whether or not oil attaches on the bar. Or, in the case of a large-sized tank having a capacity of about 30 kl, the tank is formed into a double-structure, and liquid leakage is detected by filling ethylene glycol between the two tanks and checking change in liquid surface level by a measuring cylinder.
These methods have problems in that quick detection of leakage cannot be attained and that the tank becomes expensive.